Infrasound
Definition
Sound waves too low for human hearing — below about 20 Hz — produced by earthquakes, volcanoes, and large machinery.
Unit: Hz
Technical definition
Acoustic waves with frequency below the human hearing threshold (<20 Hz). Very long wavelengths travel great distances with little absorption; elephants and whales communicate with infrasound.
Example
Elephants rumble at 14–24 Hz to communicate over kilometers — below what humans can hear but felt as much as heard up close.
Common confusion
Infrasound can be powerful despite being inaudible — low frequency does not mean low energy. Volcanic infrasound can circle the globe, and very loud infrasound can still affect the body.